A Convenient Truth: 90% Of The Tunas Are Gone!

Last week, I had the privilege of presiding over the defense of the Ph.D. dissertation of Maria José Juan-Jordá in La Coruña, Spain. Maria José is a bright young researcher and has already published several chapters of her dissertation in the peer-reviewed literature.

The first chapter in her dissertation1 is the last in a series of peer-reviewed scientific papers that demonstrate that the combined biomass of large oceanic predators (mostly tunas) has not declined by 90% as stated in another scientific paper ten years ago.Tuna in the Pacific.

The 90% decline figure came from an analysis published in 2003 which concluded that “large predatory fish biomass today is only about 10% of pre-industrial levels” (Myers and Worm, 20032). Those analyses relied heavily upon catch rates from a single fishing gear type (longline) and aggregated catch across species to estimate trends in “community biomass”. The paper quickly became high-profile in the tuna world. Many environmental groups embraced it as proof that all tunas, not just the bluefins, were in serious trouble. On the other hand, tuna scientists who were actually conducting stock assessments, especially for tropical tunas knew immediately that the 90% number was totally wrong.Over the next few years, a number of peer reviewed publications3,4,5,6,7,8 showed that the conclusions in Myers and Worm (2003) were fundamentally flawed. Two of the most important reasons for this are: The aggregation of data, and the use of data from a single fishing method. The end result is that the 90% decline an overestimate. This process of rebuttal is a natural part of the way science develops. Sometimes scientists reach conclusions that are wrong, for whatever reason, and other scientists discover flaws and point to them. A paper, once published, is not necessarily immortal.Nevertheless, the notion of 90% global demise of tuna populations is still out there. It is repeated in many consumer guides published by various environmental groups that want to influence market preferences. It also pops up elsewhere: Earlier this year, I visited the web site of a newly-formed commission that aims to improve governance of ocean resources, and I was surprised to see the 90% number mentioned there. A colleague of mine who also noticed it said he was “disappointed that one of the most rebutted fisheries paper of all time continues to raise its head.”Back to Maria José’s first thesis chapter: She and her coauthors painstakingly collected stock assessment results from 26 populations of tunas and their close relatives, mackerels and Spanish mackerels. Unlike the Myers and Worm (2003) paper, stock assessments do not rely upon a single type of data. They combine all available data from all fisheries acting upon a stock, plus information about each species’ biology. The analyses of this comprehensive data set shows that, between 1954 and 2006, the total adult biomass of these species declined by 52%. The analyses also highlight variation in the degree of depletion, depending on the biology of each species and its commercial value: The steepest declines are for the largest longest-lived highest-value temperate tunas (bluefins) and the smaller short-lived mackerels. The remaining populations, mostly tropical tunas have been fished down to approximately maximum sustainable yield (MSY) levels (for most tunas, MSY occurs at a biomass around 30% to 50% of the unfished level).So, I wonder: Why is the 90% figure still being used? Is it because it is a convenient number for fund-raising campaigns? A convenient truth? I think so. But tuna stocks and fisheries need to be managed, and each stock and each fishery has its particular issues that need to be addressed. I believe that these groups that hang on to simplistic (and erroneous) numbers are doing fisheries management a disservice. Management needs to be science-based, not faith-based. I urge these groups to stop diverting attention from the real issues and join us instead in calling upon tuna RFMOs to adopt stock-specific reference points for management and to improve their decision-making to manage each and every tuna fishery adequately.